Originally, Naval war was fought by ramming other ships and then boarding them. In Ancient Greece and Rome, the ships were usually powered by oars. Then, during the Middle Ages, the cannon was invented. These could be shot at other ships from a great distance. Ships around this time also began to use sails instead of oars for power. In the 1800s, ships began to be plated in armor for protection. Steam engines allowed them move while carrying the armor, and better ones soon made them much faster. These early warships, called ironclads, are thought to have begun the type of Naval warfare used today. Ships soon became much larger.